Caption: Nuclear reactor core of the Advanced Test Reactor (ATR) at the Idaho National Engineering and Environmental Lab (INEEL) at Idaho Falls, USA. This large reactor has been in use since 1967. It operates with a thermal power of 250,000 kilowatts. The reactor is water-cooled and the blue glow results from Cerenkov radiation, emitted when energetic charged particles travel faster through the water than light. This reactor is used for research into the effects of radiation on materials, and for the production of rare and valuable medical and industrial isotopes.